Spinal fusion cage with lordosis correction

ABSTRACT

Intervertebral cages, and methods of introducing the same, require few, if any, structural components within the cage itself, thereby providing a maximum volume for bone-graft filler. Cages according to the invention may passively expand to fill a volume within the disk space, or active expansion means may be provided. In terms of a passive expansion, at least a portion of the frame may be composed of a shape-memory material, causing the cage to naturally expand from the compressed to the expanded state once the cage is positioned within the intervertebral space. In either case, locking means are provided to maintain the shape of the cage once in position. An alternative embodiment includes first and second components, each having a compressed state which consumes a compressed volume associated with insertion of the component into an intervertebral space, and an expanded state which consumes a greater volume when the component is positioned within the intervertebral space. In terms of an active expansion, an externally accessible mechanism may be used to increase the anterior height once the cage is positioned within the intervertebral space. In all embodiments, connection means may be provided for interconnecting a plurality of cages together within the same intervertebral space.

REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. provisionalapplication Serial No. 60/148,913, filed Aug. 13, 1999, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to spinal surgery and, inparticular, to spinal cages of the type used in fusing adjacentvertebrae.

BACKGROUND OF THE INVENTION

[0003] Eighty-five percent of the population will experience low backpain at some point. Fortunately, the majority of people recover fromtheir back pain with a combination of benign neglect, rest, exercise,medication, physical therapy, or chiropractic care. A small percent ofthe population will suffer chronic low back pain. The cost of treatmentof patients with spinal disorders plus the patient's lost productivityis estimated at 25 to 100 billion dollars annually.

[0004] Seven cervical (neck), 12 thoracic, and 5 lumbar (low back)vertebrae form the normal human spine. Intervertebral discs residebetween adjacent vertebra with two exceptions. First, the articulationbetween the first two cervical vertebrae does not contain a disc.Second, a disc lies between the last lumbar vertebra and the sacrum (aportion of the pelvis).

[0005] Motion between vertebrae occurs through the disc and two facetjoints. The disc lies in the front or anterior portion of the spine. Thefacet joints lie laterally on either side of the posterior portion ofthe spine. The osseous-disc combination of the spine coupled withligaments, tendons, and muscles are essential for spine function. Thespine allows movement (flexion, lateral bending, and rotation), supportsthe body, and protects the spinal cord and nerves.

[0006] The disc changes with aging. As a person ages the water contentof the disc falls from approximately 85 percent at birth to 70 percentin the elderly. The ratio of chondroitin sulfate to keratin sulfatedecreases with age. The ratio of chondroitin 6 sulfate to chondroitin 4sulfate increases with age. The distinction between the annulus and thenucleus decreases with age. These changes are known as discdegeneration. Generally disc degeneration is painless.

[0007] Premature or accelerated disc degeneration is known asdegenerative disc disease. A large portion of patients suffering fromchronic low back pain are thought to have this condition. As the discdegenerates, the nucleus and annulus functions are compromised. Thenucleus becomes thinner and less able to handle compression loads. Theannulus fibers become redundant as the nucleus shrinks. The redundantannular fibers are less effective in controlling vertebral motion. Thedisc pathology can result in: 1) bulging of the annulus into the spinalcord or nerves; 2) narrowing of the space between the vertebra where thenerves exit; 3) tears of the annulus as abnormal loads are transmittedto the annulus and the annulus is subjected to excessive motion betweenvertebra; and 4) disc herniation or extrusion of the nucleus throughcomplete annular tears. Disc herniation can also cause arthritis of thefacet joints which, in turn may cause back pain.

[0008] The problems created by disc degeneration, facet arthritis, andother conditions such as spondylolysis, spondylolisthesis, scoliosis,fracture, tumor, or infection are frequently treated by spinal fusion.Such problems may include pain in the back or legs, nerve injury, riskof future nerve injury, or spinal deformity. The goal of spinal fusionis to successfully “grow” two or more vertebrae together. To achievethis, bone from the patient's body (spine or iliac crest) or fromcadavers, is grafted between vertebrae. Alternatively, bone graftsubstitutes, such as hydroxy appetite and bone morphogenetic protein,may be used. The bone graft is placed between the vertebrae in the discspace and/or over the posterior elements of the vertebrae (lamina andtransverse processes). The surgeon scrapes the vertebrae to createbleeding. Blood flows into the bone graft. The scraped bone, blood clot(hematoma), and the bone graft simulates a fracture. As the patientheals, the “fracture” causes the vertebrae to be fused and healtogether.

[0009] Spinal instrumentation, including cages, is placed onto or intothe spine to immobilize the vertebrae that are going to be fused.Immobilization leads to a higher fusion rate and speeds a patient'srecovery by eliminating movement. Existing cages are typically hollowmetal or carbon fiber devices placed into the disc space. Often cageshave treads, grooves, teeth or spikes that engage the vertebralendplates. The hollow center is filled with bone graft. The sides of thecages adjacent to the vertebral endplates contain holes to allow bonegrowth from one vertebrae through the cage to the adjacent vertebra. Thebone graft acts as a bridge for bone growth. Cages immobilize thevertebrae and maintain the separation between the vertebrae, a functionof the former disc material. Cages are placed into the disc space afterexcising a portion of the disc annulus and most of the nucleus. One ortwo cages may be inserted at each level.

[0010] Cages may be placed into the disc space from an anterior or aposterior approach to the spine. Cages may be combined with rods,screws, hooks, or plates. Combining cages with other instrumentationyields a stiffer construct and presumably increases the chance of asuccessful fusion. If cages are placed from an anterior approach(through the abdomen) the patient must undergo surgery through a secondincision over the back (posterior approach) if the surgeon wants to alsoinsert rods and screws. To avoid two incisions and increased patientmorbidity, many surgeons prefer to insert the cages from a posteriorapproach. Rods and screws can be added through the same incision.

[0011] The cages currently available for insertion from a posteriorapproach have a few important weaknesses. Since most cages are insertedinto the disc space in their final size and shape, the cages must belarge enough to extend from vertebra-to-vertebra. Furthermore, the cagesmust be wide enough to provide stability and provide adequate surfacearea for the vertebrae to “grow” together. Large cages insertedposteriorly risk nerve injury (from retracting the nerves to insert thecage or the edge of the cage during insertion) or extensive boneremoval. A portion of the vertebra (lamina, a portion of the facet(s)and/or the entire facet(s)) is removed to allow cage insertion. Largecages require more bone removal. Some surgeons remove one or both facetjoints to safely insert the cage or cages. Removal of the facet jointsdestabilizes the spine, which may lead to cage migration and/orpseudoarthrosis (failure of the bone to heal). Either complication mayrequire the patient to undergo additional surgery. Revision proceduresare more difficult after removal of the facet joints.

[0012] Most of the cages inserted posteriorly have parallel superior andinferior surfaces. The endplates of the vertebrae form the superior andinferior limits of the disc space. The endplates are not typicallyparallel in the lumbar spine. Generally, the endplates become closertogether as one proceeds toward the posterior portion of the disc space.This alignment creates the normal anterior to posterior curvature of thelumbar spine known as lordosis. Cages with parallel superior andinferior surfaces either fit tightly posterior and loosely anteriorly,or require removal of additional endplate posteriorly. Alternatively,the vertebrae can be fused without lordosis. None of these choices areideal.

SUMMARY OF THE INVENTION

[0013] This invention improves upon the existing art by providingintervertebral cages, and methods of introducing the same, which requirefew, if any, structural components within the cage itself, therebyproviding a maximum volume for bone-graft filler materials.

[0014] According to a first preferred embodiment, a cage according tothe invention features a compressed state associated with insertion ofthe cage into an intervertebral space having an anterior height which isless than or equal to the posterior height, and an expanded state,wherein the anterior height is greater than the posterior height toaccount for lordosis. Means are further provided for locking the cageinto the expanded state once the cage is positioned within theintervertebral space. Such means preferably includes a ratchet mechanismassociated with the anterior portion which locks the cage into desiredwedge configuration.

[0015] Cages according to the invention may passively expand to fill avolume within the disk space, or active expansion means may be provided.In terms of a passive expansion, at least a portion of the frame may becomposed of a shape-memory material, causing the cage to naturallyexpand from the compressed to the expanded state once the cage ispositioned within the intervertebral space. Alternatively, superior andinferior members may be attached to the posterior portion so as to forma spring, which causes the cage to naturally expand from the compressedto the expanded state. The superior and inferior surfaces preferablyinclude protrusions for frictionally engaging with vertebral endplatesdefining the disk space.

[0016] In terms of an active expansion, an intervertebral cage accordingto the invention may include an externally accessible mechanismoperative to increase the anterior height once the cage is positionedwithin the intervertebral space. For example, such a mechanism mayinclude a slug positioned on a threaded rod which moves posterior toanterior when the rod is rotated. An alternative embodiment includesfirst and second components, each having a compressed state whichconsumes a compressed volume associated with insertion of the componentinto an intervertebral space, and an expanded state which consumes agreater volume when the component is positioned within theintervertebral space. In this case, the shape of the components is suchthat the first component when expanded leaves a gap which is closed bythe second component when expanded. In all embodiments, connection meansmay be provided for interconnecting a plurality of cages together withinthe same intervertebral space.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a drawing which shows a preferred expandable cageaccording to the invention without bone-graft filler;

[0018]FIG. 2 is a drawing which shows the cage of FIG. 1 includingbone-graft filler;

[0019]FIG. 3 is a drawing of the cage of FIGS. 1 and 2 illustrating theuse of a radiolucent sleeve surrounding the cage;

[0020]FIG. 4 is a side-view drawing illustrating the way in which a meshmay be attached to the side openings of an expandable cage;

[0021]FIG. 5A is a drawing which shows how a screen may be suturedaround a cage;

[0022]FIG. 5B is a drawing which shows the sutured sleeve tightenedaround the cage;

[0023]FIG. 6A is a side-view drawing which shows how superior andinferior members may cooperate to enclose a cage volume;

[0024]FIG. 6B is an end-view drawing of the configuration of FIG. 6A;

[0025]FIG. 7 is a drawing which shows how mesh as opposed to solid sidesmay be used in an overlapping structure;

[0026]FIG. 8A is a drawing which shows a shape-memory cage prior tocompression for introduction within an intervertebral space;

[0027]FIG. 8B shows the device of FIG. 8A having been compressed forinsertion through an annular wall;

[0028]FIG. 8C is a drawing which shows the device of FIGS. 8A and 8Bre-expanded to fill an intervertebral in a manner consistent withlordosis;

[0029]FIG. 8D is a drawing which shows one way in which an elongatedmember may be used to hold a cage according to the invention in acollapsed state during introduction of the device into an intervertebralspace;

[0030]FIG. 9A is an oblique drawing which shows two cages beinginterconnected with a transverse bar;

[0031]FIG. 9B is a head-to-foot drawing showing the interconnected cagesof FIG. 9A with bone graft material filling the cages in the spacebetween the cages;

[0032]FIG. 10A is a drawing which begins a sequence of illustrationsshowing how two cages may be introduced side-by-side within anintervertebral space;

[0033]FIG. 10B is a drawing which shows the nucleus having beenevacuated from the disc space;

[0034]FIG. 10C shows how the vertebral endplates are scraped prior toinsertion of cages;

[0035]FIG. 10D illustrates the use of a distracter tool to hold thevertebrae apart for introduction of an adjacent cage;

[0036]FIG. 10E how the adjacent cage is introduced with the distracterin place;

[0037]FIG. 10F illustrates two cages side-by-side within anintervertebral space but not connected together;

[0038]FIG. 10G illustrates the way in which medial and lateral cages maybe connected with a transverse bar;

[0039]FIG. 11A illustrates the use of hinges as opposed to ashape-memory or spring configuration;

[0040]FIG. 11B shows the device of FIG. 11A ratcheted into a preferredshape orientation;

[0041]FIG. 12A is a drawing which shows how active means in the form ofa threaded rod and slug may be used to expand a cage according to theinvention;

[0042]FIG. 12B shows the cage of FIG. 12A having been expanded;

[0043]FIG. 13A is a drawing which begins a sequence of diagrams showinga two-part intervertebral cage and method of introducing the componentsinto a disc space;

[0044]FIG. 13B illustrates a first component having been inserted andexpanded;

[0045]FIG. 13C shows the use of an inserter to install the secondcomponent of this two-part system;

[0046]FIG. 13D shows the second component expanded to fill a gap in thefirst part;

[0047]FIG. 13E illustrates two cages side-by-side according to thisaspect of the invention;

[0048]FIG. 13F shows how one of the components preferably includes anaperture through which bone graft material is inserted;

[0049]FIG. 13G is a drawing which shows two cages inserted from ananterior approach with two transverse connectors and a radiolucentscreen holding bone graft between the cages; and

[0050]FIG. 14 is a drawing of yet a further alternative cage accordingto the invention, in this case assuming the form of a collapsible,expandable strip of material without optional multiple perforations forthe introduction of bone graft material.

DETAILED DESCRIPTION OF THE INVENTION

[0051]FIG. 1 illustrates, from an oblique perspective, a spinal cage 100according to the invention prior to the introduction of bone graftmaterial. FIG. 2 is a drawing of the device of FIG. 1, but with bonegraft material 200 introduced within the interior of the cage. Thedevice overall assumes the form of a generally rectangular cage havingapertured upper and lower members with superior and inferior surfaces102 and 104, respectively, and open sides. The device as shown isoriented with its posterior portion 110 facing generally outwardly andto the right of the page, and with the anterior portion 112 facinggenerally into and leftward on the page. The superior and inferiorsurfaces preferably include grooves 113, as shown, or alternativesurfacing to better engage with the endplates of the adjacent vertebraefor long-term stability. The superior and inferior surfaces may also beconvex to better conform to endplate concavity. Note further that theeither or both of the upper and lower member may be wider posteriorly(i.e., at 114) relative to the anterior thickness (i.e., at 116) toprovide added strength in compression at the back of the spine. A screwhole 118 is preferably provided for temporary coupling to an inserter.The screw hole is also preferably recessed to accommodate a transverseconnector bar without rotation, as described further below.

[0052] When the device is implanted within the intervertebral space, theanterior sections of the upper and lower members are passively oractively expanded as described below to fill the space between theadjacent vertebrae in a manner consistent with lordosis at that sectionof the spine. In the preferred embodiment, the anterior portion 112includes mating members 120 and 122 with teeth 124 or other features toform a locking or ratchet mechanism, as shown. Whatever apparatus isused, the purpose is to maintain the height of the anterior portion ofthe cage at a desired level consistent with lordosis upon installation.

[0053] As shown in FIG. 3, a radiolucent mesh sleeve 300 is alsopreferably installed around the cage when filled with bone graftmaterial. FIG. 4 is a side-view drawing which illustrates a preferredtechnique for attaching a radiolucent screen to the side of the cage.Preferably the screen, of a nylon mesh construction, is attached to thecage through the use of a nylon suture 402 using a plurality ofspaced-apart holes 404 positioned around the side opening. Instead of,or in conjunction with, the screen attachment just described withreference to FIG. 5A, a mesh sleeve or bag 502 may wrapped around thecage 504 with the edges or opening sewn together, as shown at 506. FIG.5B is a drawing which shows the sleeve or bag tightened around the cage.As opposed to the use of a screen material, the cage may be constructedsuch that a portion of the top or bottom half sleeves over the otherhalf, thereby closing the side openings when the two halves are puttogether. Such an arrangement is shown in FIGS. 6A and 6B. As opposed tosolid side members, overlapping mesh panels may be used, as shown inFIG. 7.

[0054] Either active or passive means may be used to expand the cagewhen placed. In terms of passive approaches, the cage itself ispreferably fashioned as a spring or constructed of a shape-memorymaterial allowing the device to be compressed for implantation through arelatively small opening, but, once in place, the material naturallyreturns to its pre-implantation configuration. With respect to thepreferred embodiment of a cage with lordosis correction, the devicerelaxes or expands into a wedge which is taller anteriorly relative toits posterior height. As such, the device may be introduced with theheight of the anterior section of the device being less than or equal tothe height of the posterior section but, once in place and with thedevice resuming its pre-installation shape, the anterior height isgreater than the posterior height.

[0055] Various materials may be utilized as part of a cage whichpassively expands into a desired shape according to the invention. Forexample, a list of candidate materials would include stainless steel,cobalt-chromium-molybdenum alloys, titanium, carbon reinforced polymers,shape memory alloys, materials commonly used for commercial andindustrial springs, and nylon or other fiber or polymer materials used for the radiolucent screen or mesh sleeve. In terms of carbon-reinforcedpolymers, those materials referenced in U.S. Pat. No. 5,192,327, theentire contents of which is incorporated herein by reference, may beused, which include polyetherether ketone, (polyether ketone, etherketone, and ketone), polycarbonate, polypropylenes, polyethyelene andpolysulfone type plastic materials with or without glass or carbonfibers. In terms of shape-memory alloys, those materials set forth inU.S. Pat. No. 5,954,725, the entire contents of which are incorporatedherein by reference, may be used, including, but not limited to alloysof copper and zinc, nickel titanium, silver and cadmium, and othermetals and materials, including Nitinol RTM).

[0056] As discussed above, apparatus is provided at the anterior portionof the device to lock the anterior height once a desired wedge shape isachieved. FIG. 8A shows a device according to the invention having ashape memory configuration which will be naturally resumed following atemporary compression. This temporary compression is shown in FIG. 8B,wherein the anterior portion is compressed into a generally rectangularshape for introduction into the intervertebral space. Following suchintroduction, the device resumes it memorized shape, as shown in FIG.8C. Once a desired wedge configuration has been achieved, the lockingmechanism at the anterior portion of the device engages once the desiredanterior height has been achieved, thereby preventing furthercompression of the cage overall.

[0057]FIG. 8D illustrates, from an oblique perspective, instrumentationwhich may be used to introduce a cage according to the invention into anintevertebral space in a collapsed condition. Such instrumentation mayinclude a central elongated member 880, having a distal end attached tothe posterior portion of the cage as through the threaded hold 118 shownin FIG. 1. The proximal end of the member 880 would include in this casea plate 882 having an aperture which receives a second elongated member883. The second member 883 extends through the posterior section of thedevice, and attaches one of the interlocking forward members at 884.Using such an instrument, the surgeon would pull back on rod 883 whileholding rod 880, allowing the anterior portion of the cage to benon-ratcheted and reduced in size. Following introduction of the cageinto the intervertebral space, rod 883 may be removed or pushed forward,depending upon cage construction, causing the anterior portion to expandas appropriate and lock into ratcheting position. Following thisexpansion, both members 880 and 883 would be removed, leaving only thecage in place. It is noted that such a system is applicable not only tothe embodiment of the type shown in FIG. 1, but also to those depictedin FIGS. 11 and 12.

[0058]FIGS. 9A and 9B illustrate the way in which two devices accordingto the invention may be interlocked using a transverse connector 902 andfasteners such as screws 904. The transverse connector 904 fits intorecesses 906 and 908 which are preferably provided on the posteriorsurfaces of the devices, to prevent rotation and provide a more planarposterior surface overall, as discussed above. Note that the devices 910and 920 may be entirely independent of one another in terms of theirfinal shape, such that they need not assume the same wedge size onceimplanted, but rather, may be different in accordance with spinaldefects and other aspects of patient physiology. FIG. 9B is a top viewof two devices 910 and 920 installed within the intervertebral space,having been joined with a transverse connector 902. Openings 930 and 932in the wall of the annulus 940 show where the devices were installedmedially and laterally. FIG. 9B also shows how bone graft material 950is preferably introduced throughout the intervertebral space, not onlywithin the cages themselves, but in between the cages, as shown.

[0059]FIG. 10A begins a sequence of drawings which help to illustratehow a pair of spinal cages according to the invention are implanted andadjusted. FIG. 10A illustrates, in simplified schematic form, thelocation of openings formed 1002 and 1004 in the annulus 1006, therebygaining access to the nucleus 1008. FIG. 10B is a drawing which showsthe nucleus having been evacuated from space 1010. FIG. 10C is aposterior-anterior view in partial cross-section illustrating how theend plates of the adjacent vertebrae are scraped at 1012 and 1014 usingtools inserted through the holes in the annulus fibrosis.

[0060] In FIG. 10D, a distracter tool 1020 is introduced medially orlaterally through one of the holes in the annulus, so as to separate theadjacent vertebrae a desired amount, as determined through existingsurgical techniques. With the distracter in place, a cage according tothe invention is inserted into the other opening in the annulus, asshown in FIG. 10E. FIG. 10F is a drawing which shows two cages in placewithout a transverse connector, and FIG. 10G is a drawing which showsthe cages of FIG. 10F coupled together using a transverse connector1030.

[0061] As opposed to the entire use of shape-memory material, hinges maybe used instead of, or in conjunction with shape-memory elements. Forexample, as shown in FIG. 11A, one or more hinges 1102 may be providedbetween the upper member 1104 and posterior portion 1106 and/or theupper member 1104 and a portion of the ratchet mechanism 1112, as shown.Springs 1110 and/or 1112 may also be provided to bias the variousmembers so as to expand appropriately upon introduction in to the diskspace. FIG. 10A shows the device in the collapsed state, such that theanterior portion is actually reduced in height as compared to theposterior portion, whereas, in the expanded device, the anterior portionis equal to, or greater than, the posterior portion as a function oflordosis, as shown in FIG. 11B.

[0062] As shown in FIG. 12A, adjustment mechanisms may also be providedaccording to the invention to achieve a desired wedge configuration onethe cage is in place using an externally applied adjustment force. InFIG. 12A, the cage is shown in a collapsed state with a slug 1202 on athreaded rod 1204 being set back from the anterior region, allowing theheight of the device to be compressed from front to back, as shown. Byturning the threaded rod 1204, the slug 1202 is moved to posterior toanterior, with the surfaces of the slug bearing against the lower andupper members, causing the upper member to hinge at point 1206, and moveupwardly, as shown in FIG. 12B, to assume a desired wedge-shapedorientation following introduction into the intervertebral disc space.

[0063]FIG. 13A is a drawing viewed from head to foot of an alternativeembodiment of the invention utilizing a shape-memory cage which isconstructed within the disc space, but which does not necessarilyprovide for lordosis correction. Each cage is preferably constructed ofan anterior shape memory component 1302 and a posterior component 1304,also preferably constructed in the form of a shaped-memory device. FIG.13A illustrates the anterior component 1302 being inserted according tothis embodiment. FIG. 13B shows the device 1302 in its expanded orrelaxed configuration, assuming the general form of a “C” with theopening of the C being oriented posteriorly when in place.

[0064]FIG. 13C shows the posterior component 1304 in a collapsed stateand being pushed outwardly through the distal opening of an inserter1306. FIG. 13D is a top-view drawing showing the inserter removed,allowing the anterior shape-memory component to relax and resume aconfiguration which blocks the opening of the C-shaped anteriorcomponent. The procedure is then repeated for the other side, resultingin adjacent assemblies. FIG. 13E is a drawing which shows two cagesaccording to the invention placed side by side within a common diskspace, each cage having separate anterior and posterior components.Preferably, the posterior components include an aperture through whichbone-graft material may be introduced into the cages, then plugged oncefilled, as shown in FIG. 13F. As with the generally rectangular cagedescribed with reference to the preferred embodiment of the invention,the adjacent assemblies may be joined using one or more transverseconnectors, as shown in FIG. 13G, and the space between the assembliesmay be filled with additional bone-graft material, thereby substantiallyfilled the space between the vertebral endplates.

[0065] According to a different configuration, as an alternative to atwo-part collapsible strip system of the type just described, a singlecollapsible/expandable strip may be used, as shown in FIG. 14.Preferably, the device would be collapsed into an elongated shape forintroduction through an inserter, after which, being composed of aspring or shape-memory material, would expand or relax into a largervolume. Preferably, such a device would include a plurality ofapertures, as shown in FIG. 14, such that one or more may be used forthe introduction of bone graft material following implantation andexpansion.

I claim:
 1. An intervertebral cage, comprising: a rigid frame havinganterior and posterior portions, superior and inferior surfaces, and twosubstantially open sides; the thickness of the cage between the superiorand inferior surfaces in the anterior portion defining an anteriorheight, and the thickness of the cage between the superior and inferiorsurfaces in the posterior portion defining a posterior height; the cagehaving a compressed state associated with insertion of the cage into anintervertebral space, wherein the anterior height is less than or equalto the posterior height, and an expanded state, wherein the anteriorheight is greater than the posterior height to account for lordosis; andmeans for locking the cage into the expanded state once the cage ispositioned within the intervertebral space.
 2. The intervertebral cageof claim 1, wherein the means for locking the cage into the expandedstate includes a ratchet mechanism associated with the anterior portion.3. The intervertebral cage of claim 1, wherein the interior volume ofthe frame is substantially unoccupied to receive bone graft material. 4.The intervertebral cage of claim 1, wherein at least a portion of theframe is composed of a shape-memory material causing the cage tonaturally expand from the compressed to the expanded state once the cageis positioned within the intervertebral space.
 5. The intervertebralcage of claim 1, wherein the superior and inferior surfaces are attachedto the posterior portion so as to form a spring which causes the cage tonaturally expand from the compressed to the expanded state once the cageis positioned within the intervertebral space.
 6. The intervertebralcage of claim 1, wherein the superior and inferior surfaces includeprotrusions for frictionally engaging with vertebral endplates definingthe disk space.
 7. The intervertebral cage of claim 1, wherein one orboth of the superior and inferior surfaces are convex.
 8. Theintervertebral cage of claim 1, wherein one or both of the superior andinferior surfaces are hinged to the posterior portion.
 9. Theintervertebral cage of claim 1, wherein the means for locking the cageinto the expanded state includes one or two anterior members hinged toone or both of the superior and inferior surfaces, respectively.
 10. Theintervertebral cage of claim 1, further including an externallyaccessible mechanism operative to increase the anterior height once thecage is positioned within the intervertebral space.
 11. Theintervertebral cage of claim 10, wherein the externally accessiblemechanism includes a slug positioned on a threaded rod which movesposterior to anterior when the rod is rotated.
 12. The intervertebralcage of claim 1, further including a radiolucent screen covering atleast the open sides of the cage.
 13. The intervertebral cage of claim1, further including connection means associated with the posteriorportion for interconnecting a plurality of cages together within thesame intervertebral space.
 14. An intervertebral cage, comprising: firstand second components, each having a compressed state which consumes acompressed volume associated with insertion of the component into anintervertebral space, and an expanded state which consumes a greatervolume when the component is positioned within the intervertebral space;the shape of the components being such that the first component whenexpanded leaves a gap which is closed by the second component whenexpanded.
 15. The intervertebral cage of claim 14, wherein the firstcomponent is positioned generally anteriorly within the intervertebralspace, and the second component is positioned generally posteriorlywithin the intervertebral space.
 16. The intervertebral cage of claim14, wherein at least one of the first and second components includes asidewall with an aperture to receive bone-graft material therethrough.17. The intervertebral cage of claim 14, wherein one or both of thefirst and second components is composed of a shape-memory materialenabling that component to naturally expand from the compressed to theexpanded state once the cage is positioned within the intervertebralspace.
 18. The intervertebral cage of claim 14, wherein one or both ofthe first and second components form a spring enabling that component tonaturally expand from the compressed to the expanded state once the cageis positioned within the intervertebral space.
 19. The intervertebralcage of claim 14, further including a radiolucent screen covering atleast the open sides of the cage.
 20. The intervertebral cage of claim14, further including connection means associated with the posteriorportion for interconnecting a plurality of cages together within thesame intervertebral space.